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tographically by liquefaction or volatilization; and no transparent, solid being rendered cheroically opaque by change of form. Hence it is obvious that this opacity or transparency is intimately connected with the atomic or chemical character of the body, and not merely with its state of aggregation. Although the absorption of the chemical rays varies greatly in the different gases, which therefore in this action display an analogy to their effects upon radiant heat, yet those gases which absorb the rays of heat most powerfully are often highly transparent to the chemical rays, as is seen in the case of aqueous vapour, of carbonic acid, cyanogen, and olefiant gas, all of which are compound substances, not chemical elements. These observations were considered by the speaker as opposed to the view of Dr. Tyndall, that compounds, as compounds, act more energetically than elementary bodies in absorbing the heat-rays, owing to the greater inertia of the particles of compounds. Any physical cause of this kind ought, however, to enable such bodies to act equally as absorbents of the luminous and chemical rays.
In the case of reflection from polished surfaces, the metals were found to vary in the quality of the rays reflected ; gold and lead, although not the most brilliant, reflecting the rays more uniformly than the brilliant white surfaces of silver and speculum metal.
2. Photographic Spectra of the Elements.—But the most interesting results are those obtained by examining the spectra produced by varying the nature of the metallic electrodes employed as terminals to the secondary wires of the induction coil. Professor Wheatstone showed twenty-eight years ago that the visible spectrum of each metal is perfectly characteristic when electromagnetic sparks are transmitted between two surfaces of the metal; and the same thing is equally true of the invisible portion of the spectrum.
Even the various gaseous media become so intensely heated by the passage of the electric spark, that they furnish photographic spectra, each of which is characteristic of the body which occasions it; and when the electric discharge of the secondary coil becomes intensified by use of the Leyden jar, the sparks not only produce the spectra due to the metals, but to the gaseous medium in which the electrodes are immersed ; so that a mixed spectrum is the result. The spectra produced by the metals are characterized by bands, of which the extremities only are visible; whilst the gaseous spectra yield continuous lines, which traverse the whole width of the spectrum. When a compound gas is made the medium of the electric discharge, the spectra produced are those of the elementary components of the gas, It seems as though at these intense temperatures chemical combinations were impossible ; and oxygen and hydrogen, chlorine and the metals probably might all coexist in a separate form, though mechanically intermingled.
The spectrum produced by the ignition of a solid or a liquid always yields a continuous band of light, containing rays of all degrees
of refrangibility; but the same body, when converted into vapour, usually produces a spectrum consisting of a series of bright bands of particular colours, separated from each other by intervals more or less completely dark, gaseous bodies emitting rays of certain definite refrangibility only.
From the striped character of the photographic spectra, it is obvious that the vibrations are emitted from the different metals in the form of vapour, and not merely in that of detached particles projected from the electrodes by disruptive discharge.
This observation may give some idea of the intensely high temperature attained by the spark ; since it is uniformly observed that the higher the temperature, the more refrangible are the vibrations. We are, indeed, furnished in this case with a rude, but still, under the circumstances, with a valuable pyrometric means of estimating these exalted temperatures.
To give an illustration of the mode of applying the observation : The hottest wind-furnace of ordinary construction yields a temperature probably not much exceeding 4,500° F. By calculations founded upon the amount of heat ascertained by Andrews and others to be emitted during the combustion of a given weight of hydrogen, and the experiments of Regnault upon the specific heat of oxygen, hydrogen, and steam, it has been shown by Bunsen that the temperature of the oxyhydrogen flame cannot exceed 14,580°F. The speaker stated that he had obtained spectra by introducing lime and sulphate of magnesia into the oxyhydrogen jet : these incombustible materials therefore could not be heated by the burning gases to a higher point than 14,600° F., but the spectra so obtained coincide in their photographic lengths with that of the solar spectrum. Hence, the temperature of the sun may be approximatively estimated to be not higher than that of the oxyhydrogen flame. It certainly appears to be far below that of the electric spark. Magnesium in the electric spark gives a remarkably strong band just beyond the limits of the solar spectrum. Now magnesium is as clearly proved to exist in the solar atmosphere as any element, if we be admitted to have any such proof at all. But inasmuch as the special band which characterizes magnesium at a high temperature in the electric spark is wanting in the solar spectrum, it is difficult to avoid the conclusion that the temperature of the solar atmosphere is below that generated by the electric spark,
The speaker then adverted to Kirchhoff's well-known theory of the origin of Fraunhofer's dark lines in the solar spectrum, based on the observation that when any substance is heated or rendered luminous, rays of a definite refrangibility are given out by it; whilst the same substance, vapour, or gas, has the power of absorbing rays of this identical refrangibility. Now Kirchhoff supposes that in the luminous atmosphere of the sun the vapours of various metals are present, each of which would give its characteristic system of bright lines; but behind this incandescent atmosphere containing metallic vapour is the still more intensely heated solid or liquid nucleus of the sun,
which emits a brilliant continuous spectrum, containing rays of all degrees of refrangibility. When the light of this intensely heated nucleus is transmitted through the incandescent photosphere of the sun, the bright lines which would be produced by the protosphere are reversed, and Fraunhofer's black lines are therefore the reversed bright lines of which the spectrum due to the gaseous atmosphere of the sun would consist if the intensely heated nucleus were no longer there.
Froin his observations Kirchhoff concluded, by a comparison of the bright lines in the spectra of various metals with the dark lines of the solar spectruni, that potassium, sodium, magnesium, calcium, iron, nickel, chromium, manganese, and possibly cobalt, were present in the sun's atmosphere; and Ångström, continuing the examination into the blue and violet extremity, believes that he has shown the existence of hydrogen, aluminum, and possibly of strontium and barium. Diagrams were exhibited, showing some of the solar lines according to the observations of Kirchhoff and Ångström, from which it appears that B corresponds to potassium.
These observations on the solar spectrum give great interest to similar observations upon the stars, the light of which, however, is so feeble as to render the investigation of their spectra a task of no ordinary delicacy. Fraunhofer examined four or five of the brightest stars, and considered that the light of Sirius and Castor had lines differing decidedly from that of the sun. Capella and a Orionis resembled the solar light more closely. Donati has since examined several of the brighter stars, and given a drawing of some of their lines. The speaker had recently, conjointly with his friend, Mr. Huggins, been pursuing the same investigation with the excellent eight-inch equatorial refractor of the latter, and they had obtained some interesting results, having measured the principal lines in Sirius, Betelgeus, and Aldebaran, a diagram of which was exhibited, showing a more detailed spectrum of each of these stars than had been given by any previous observer. He also projected on the screen a microscopic photograph of the spectrum of Sirius that his friend and he had succeeded in obtaining. The light of this star, from the measurements of Sir J. Herschel and Mr. Bond, is little more than the one-six-thousandth-millionth part of that of the sun ; and although probably not less in size than sixty of our suns, is estimated at the inconceivable distance of more than one hundred and thirty millions of
millions of miles. And yet it is influencing, in a measure, the chemical changes which are perpetually occurring upon the earth's surface, and by suitable means the changes may be recorded, estimated, and measured—the force which was registered by the photograph having emanated from Sirius twenty-one years before !
Capella, which Admiral Smyth estimates at more than three times the distance of Sirius, also gave a photograph, when its spectrum was thrown upon a collodion plate, the effect being produced by rays, which left the star probably when the oldest person in the room was yet a boy.
[W. A. M.]
WEEKLY EVENING MEETING,
Friday, March 13, 1863.
The Rev. John Barlow, M.A. F.R.S. Vice-President,
in the Chair.
JOHN HALL GLADSTONE, Esq. Ph.D. F.R.S.
On Fogs and Fog Signals.
DURING the course of the inquiry made by the Royal Commission on Lights, Buoys, and Beacons, the attention of my colleagues and myself was called to the fog signals which form part of the apparatus of many lighthouses, and of all British light-ships. In the report we expressed our conviction, “ that they are not sufficiently powerful, and recommend the provision of a more efficient warning in fog as subject of investigation and experiment." About the same time, some scientific men in Ireland stirred in the matter, and induced the British Association to appoint a committee, at the head of which is the Rev. Dr. Robinson, of Armagh, to bring the importance of the subject more directly under the notice of the legislature. These circumstances led me to turn my attention to fog; and I propose now to lay before you some of the results arrived at, with reference both to the meteorological pbenomenon itself, and to the means adopted for preventing its disastrous consequences among the vessels that sail along our shores.
I have received voluminous returns of the occurrence of fog at about 250 stations, for which I am indebted to the kindness of the three general Lighthouse Boards,—the Trinity House, the Northern Commissioners of Lighthouses, and the Ballast Board of Dublin : also to the Board of Trade, through Admiral Fitz Roy; and to Mr.
Vol. IV. (No. 37.)
Glaisher. I wish here also to express my thanks to several gentlemen who have aided me in the preparation of this discourse, especially Mr. Alexander Cuningham, who has just read a paper on the subject at the Royal Scottish Society of Arts, and to our friend Professor Wheatstone.
Fogs. A fog is simply a cloud resting on the earth. In the first discourse of the present season, Professor Tyndall explained the formation of clouds from the aqueous vapour in the atmosphere; and defined a cumulus as “the visible capital of an invisible column of saturated air.” A fog is the capital without the column. It is the moisture evaporating from the warm earth, or river, or sea, condensed at once by the colder air. Mr. Glaisher told us here how from his lofty position in the balloon, he saw a fog following all the windings of the Thames. This is a frequent observation, and it reminded me of a scene from the summit of the Righi one morning last summer. There lay in the valley of the Reuss a mist like a white sheet on the ground, but as the sun began to exert his power, and a light breeze to spring up, the uniform layer began to break into regular masses, and soon far beneath us there stretched a cirrus cloud, identical in aspect with those we so often see in the highest regions of the atmosphere.
Fog, then, is composed of minute particles of water, most likely in a globular form, for there seems to be no ground for the popular notion of vesicles of vapour. Smoke enters largely into the composition of that peculiar yellow fog which visits London a few times each year,-a fog of wonderful darkness and quietness, and strangely bewildering.
This condensed vapour has a great effect in obstructing the passage of light; the sun himself cannot look through it. A slight mist seems to attack principally the more refrangible rays of the spectrum, so that the light appears redder than usual. I once analysed with a spectroscope the rays which reached Worthing from the great revolving light on Beachy Head, twenty-eight miles distant, and found that those only situated between Fraunhofer's lines C and F were transmitted. This was on what would be called a clear summer night. An objection has been raised against the orange-red glass used in many of the French lighthouses, that in misty weather all bright lights are reduced to very nearly that colour, and thus the distinction is lost; a misfortune that could hardly happen with the deep-red glass employed for the red lights of the British Isles. When the sun shines through a cloud or mist, we do not detect those atmospheric lines which make their appearance when his disk is near the horizon. Yet I have observed in London, when the sun at a considerable altitude loomed red through a slight fog, that the characteristic C 6, 8, and y were visible.
There is, of course, every conceivable gradation between the lightest haze and the densest fog, and it is a difficult matter to draw a line of distinction between fog and mist. The value of the meteorological